In order to realize high-density recording, a reduction in medium noise is often pursued. In order to achieve a reduction in medium noise, it is effective to reduce the exchange interaction between grains of a recording layer. Currently, granular recording layers in many perpendicular recording media have a gradation of the magnetic anisotropy (Ku) in such a way that Ku becomes smaller toward the upper layer (a graded-Ku structure). For this graded-Ku structure, the top granular layer that exhibits a low Ku has a large exchange interaction between grains. Therefore, if the exchange interaction between grains in the top granular layer can be reduced, noise will be reduced.
It is important for a perpendicular magnetic recording medium to have improved writeability and signal to noise ratio (SNR) while the thermal stability of the magnetization is maintained. Many perpendicular magnetic recording media which are commercially available have a layered structure in which the following layers are stacked in succession on a substrate: a soft magnetic underlayer; a nonmagnetic intermediate layer; a recording layer; a protective layer having a carbon overcoat; and a lubricant (which may be applied after manufacturing of the medium and delivery to the user).
As disclosed in Japanese Patent Office (JPO) Pub. Nos. 2001-23144A, 2003-91808A and 2003-168207A, for example, a recording layer may have a structure in which a granular layer having a granular structure and a ferromagnetic metal layer which does not have a clear granular structure are stacked one above the other. The granular layer may be formed by a material having high magnetic anisotropy, and has the role of enhancing thermal stability. In the granular layer, oxides are segregated at the magnetic grain boundary. As a result, the grain boundary width increases and the magnetic cluster size is reduced. Noise decreases when the magnetic cluster size is reduced. A material consisting of a CoCrPt alloy and not containing any oxides is normally selected as the material for the ferromagnetic metal layer. A material having relatively low magnetic anisotropy forms the ferromagnetic metal layer, and therefore the switching field is reduced by using this layer. The ferromagnetic metal layer therefore improves writeability. Moreover, it is known that the combination of the granular layer and the ferromagnetic metal layer makes it possible to achieve a high level of writeability and high SNR while the thermal stability is maintained.
It is known that the Ku-graded structure in the recording layer is effective for improving media performance. The incoherent mode of magnetization reversal is promoted by a Ku-graded structure. The writeability can be improved when the incoherent mode is promoted.
JPO Pub. No. 2011-14191A, for example, discloses a configuration in which the granular layer has a two- or three-layer structure and the magnetic anisotropy decreases stepwise toward the upper layer. Such a configuration promotes the incoherent mode of magnetization reversal, and therefore the thickness of the ferromagnetic metal layer may be reduced while the writeability is adequately maintained, so the magnetic cluster size may be reduced.
Furthermore, JPO Pub. Nos. 2009-187597A and 2009-110606A disclose a lower granular layer formed on the substrate side that is formed by a magnetic material having relatively high magnetic anisotropy, and the following layers are formed thereon in succession: an exchange control layer (ECL); a granular layer formed of a material having relatively low magnetic anisotropy; and a ferromagnetic layer. The inter-layer exchange coupling is controlled by the ECL and then the incoherent mode of magnetization reversal is promoted. In addition to the Ku-graded structure, the ECL promotes the incoherent mode of magnetization reversal. As a result, the writeability is further improved. International Pub. No. WO2010/038448 also discloses the same kind of configuration in another example.
In International Pub. No. WO2010/038448, an ECL is formed between the main magnetic recording layer and the ferromagnetic layer with the aim of controlling the intermediate layer exchange coupling, and a nonmagnetic CoCr alloy is used for the ECL.
A region where there is a large distribution in the crystal orientation and magnetic grain size is present in the initial layer of the recording layer due to the lattice constant mismatch with the intermediate layer, among other things. This region is one cause of increased noise. International Pub. No. WO2010/038448 and JPO Pub. No. 2007-184066A disclose methods of correcting for this, for example. If the initial layer of the recording layer is formed by a material having low saturation magnetization, it is possible to suppress the noise caused by a large distribution in the crystal orientation and magnetic grain size. A material with a high Cr concentration and a high oxide concentration is selected as the material having low saturation magnetization. The role of this layer is to promote the grain growth in the recording layer formed above, and therefore it is often referred to as an “onset layer.”
In order to achieve good read/write properties in a perpendicular magnetic recording medium, it is necessary to reduce the switching field distribution (SFD) and the magnetic cluster size, while also maintaining the writeability and surface roughness. Detailed investigations have been carried out from a number of perspectives with regard to a perpendicular magnetic recording medium in which the granular layer has at least a three-layer structure, and the following layers are stacked in succession from the substrate side: an onset layer; a high magnetic anisotropy layer (high-Ku layer); and a low magnetic anisotropy layer (low-Ku layer). The results showed that the inter-grain exchange interaction in the low-Ku layer was large, and this caused increased magnetic cluster size in the medium after stacking. Reducing the magnetic cluster size by reducing the inter-grain exchange interaction in the low-Ku layer is therefore effective for achieving good read/write properties.
The reason why the inter-grain exchange interaction in the low-Ku layer is increased is because the “onset layer” formed by a high oxide concentration plays a role for a onset of the grain growth of the recording layer. The onset layer makes it possible to increase the grain boundary width and reduce the inter-grain exchange interaction of the layer deposited on the onset layer.
However, it was found that as a result of careful observation of the cross-sectional structure using a transmission electron microscope, the region where the grain boundary width increased was only in the initial region of the recording layer nearest to the onset layer. It was also clear that, as the distance from the initial region increased, the grain boundary width decreased. The effect that the onset layer has in increasing the grain boundary becomes smaller and smaller the farther from the onset layer, such as toward the top of the recording layer.
Thus, a low-Ku layer positioned far from the onset layer results in a strong tendency for the grain boundary width to decrease. When the grain boundary width decreases, the inter-grain exchange interaction increases and the magnetic cluster size increases. In practice, investigations showed that when the above-mentioned low-Ku layer is formed on the upper layer of a high-Ku layer, the magnetic cluster size of the laminated medium is considerably increased. However, reducing the inter-grain exchange interaction of the low-Ku layer by changing the composition and deposition process of the low-Ku layer is very difficult from the point of view of writeability and surface roughness as described below.
For example, it is possible to reduce the inter-grain exchange interaction of the low-Ku layer by performing the following. First, raising the oxide concentration in the low-Ku layer in order to increase the grain boundary width; second, reducing the Cr concentration in the low-Ku layer and enhancing the magnetic anisotropy within the magnetic grains; and/or third, increasing the sputter gas pressure when the low-Ku layer is formed. However, in the first approach, the SFD increases. In the second approach, the writeability deteriorates. In the third approach, the surface roughness deteriorates. It is therefore difficult to achieve good recording characteristics with any of these conventional methods.
Therefore, it would beneficial to reduce magnetic cluster size and obtain good read/write properties without significantly changing the composition and deposition process of the low-Ku layer, e.g., while maintaining the writeability and surface roughness.